I think everything is in the title.
Nowadays, more and more people are using these two languages to write texts such as blog posts, lecture notes, essays or even reports. While there are several dedicated editors out there, sometimes coming with ODT and/or PDF export tools, I think it would be great to be able to save LO Writer documents as Markdown and/or rst.
Of course, these languages can't implement every ODF feature, but that's already the case with RTF and all the more plain text.

Not only 'Save' but full support.
Example: in Spell and Checking dialog it should not highlight ':math:`a^2` as problem pattern while using RST format text text.
A preview window/area would be also great.

+1 also.
Use case: LibreOffice Online has all the online collaborative document editing features that I need except for markdown support. Rather than having to have a dedicated stackedit or etherpad installation just for markdown files, it would be extremely convenient to be able to use LibreOffice Online for markdown as well as the other hundred plus document formats supported by LibreOffice. An export filter alone would probably be sufficient for this case.